Or, Why is this happening?

At Mod Loading Time

At Save File Load Time

• Can't load my save file. - You probably changed the name of something and a Thing in the save file can't find its definition to instantiate. You can get your file back if you edit your save file and remove any reference to definitions that you removed.

At Runtime

• The Tick callback isn't working - Make sure your XML definition has a tickerType tag
• I added a new class inheriting from ThingDef (ex. XThingDef) but it isn't showing up in the developer tools when I go to place it. - In XML, don't define it as "<XThingDef>" but as "<ThingDef Class='NAMESPACE.XThingDef'>".